1. Field of the Invention
The present invention is directed to a polarization dependent liquid crystal display using reflective spatial light modulators, and more particularly, to a display having reflective spatial light modulators with twisted nematic liquid crystal devices that exhibit polarization dependence on the incident light.
2. Discussion of the Prior Art
Increasingly, cathode ray tube (CRT) displays are being replaced with liquid crystal displays (LCDs). LCDs use spatial light modulators (SLMs) to form images. One type of an LCD is an active-matrix-driven liquid crystal display (AMLCD) that uses SLMs. The SLMs may be either transmissive or reflective.
FIG. 1 shows a basic structure of a display 100 having conventional active-matrix-driven liquid crystal transmissive spatial light modulators (AM LC SLMs). The display 100 has a back polarizer 110 and a front polarizer 115 which faces a viewer. Front and back glass substrates 120, 125 are located between, and next to, the back and front polarizers 110, 115, respectively. The AM LC SLMs, both transmissive and reflective, typically have repetitive unit cells or picture elements (pixels) 130. FIG. 1 shows 9 pixels in a 3.times.3 pixel array.
An array of transistors 135, such as field effect transistors (FETs), is formed on the back glass substrates 125. Each FET 135 in the array has its own transparent conductive electrode 140, referred to as a pixel electrode 140. Each FET 135 and pixel electrode 140 are part of one pixel (or subpixel) of an image formed on the front polarizer 115 of the display 100. Collectively, the pixel electrodes 140 form an array of pixel electrodes.
On the front substrate 120, a transparent conductive front electrode 145 is formed, which is common to all the FETs 135 in the transistor array. The front electrode 145 is referred to as a common counter-electrode. Optional color filters 150 arranged in an array may be formed between the front substrate 120 and the common counter-electrode 145. The color filters include red, green and blue filters, where each pixel has its own filter.
A liquid crystal (LC) medium 155 is sandwiched between the transparent conductive pixel and common counter-electrodes 140, 145. Back and front alignment layers (not shown), such as rubbed polyimide films, are formed between the LC medium 155, and the pixel and common electrodes 140, 145, respectively. A back-light source 160 illuminates the back of the display panel 100, where light rays 165 from the back-light source 160 are incident on the back polarizer 110.
Each FET 135 is an on/off transistor switch that supplies a voltage to the pixel electrode 140 in the ON condition. This in turn generates an electric field between the pixel electrode 140 and the common counter-electrode 145. The electric field aligns molecules of the LC medium 155. This alignment causes light passing through the LC medium 155, from the back-light source 160, to form an image on a screen (not shown) located between the front polarizer 115 and a viewer (not shown).
Instead of transmissive SLMs, where the pixel electrodes 140 are transparent, reflective SLMs may be used having reflective pixel electrodes. For reflective AM LC SLMs having reflective pixel electrodes, the transparent conductive pixel electrode 140 is replaced with a reflective metal electrode. Each metal pixel electrode of a reflective SLM typically occupies a larger area than a corresponding transparent pixel electrode 140 of a transmissive SLM. The additional area of the reflective metal pixel electrode covers the FET 135.
For reflective AM LC SLMs, there is no need for the back-light source 160 used with transmissive SLMs. Instead, ambient light or another light source illuminates the display panel from the front of the panel, e.g., from the top or front polarizer 115 shown in FIG. 1. A more detailed description of a display having reflective AM LC SLMs is given in connection with FIG. 7.
FIG. 2 shows an equivalent circuit 200 of one of the pixels 130 shown in FIG. 1. Although FIG. 1 is a display using transmissive SLMs, FIG. 2 is an equivalent circuit 200 for both transmissive and reflective SLMs. A display using reflective SLMs is shown in FIGS. 3 and 4.
As shown in FIG. 2, the gate 205 of the FET 135 is connected to a gate bus line 210, while the FET drain 215 is connected to a data bus line 220. The source 225 of the FET 135 is connected to the pixel electrode 140, which is shown in FIG. 1 as the transparent pixel electrode, and is also shown in FIGS. 3, 4 as a reflective pixel electrode 140'. The LC medium 155 of FIG. 1 is equivalent to a capacitor 230, which has one terminal connected to the pixel electrode 140 and another terminal connected to the transparent common counter-electrode 145.
A storage capacitor 240 provides parallel capacitance to the LC capacitor 230. The storage capacitor 240 is terminated on a common line 260, which is common to all the storage capacitors 240 in the display. Another alternate design for a storage capacitor is to replace the storage capacitor 240 by a storage capacitor 250, shown as dashed lines in FIG. 2, which is connected from the pixel electrode 140 to an adjacent pixel gate bus line 210'. The adjacent pixel data bus line is shown as reference numeral 220' in FIG. 2.
When a voltage below a threshold voltage is applied on the gate bus line 210, the FET 135 is in an OFF-condition (OFF state). The OFF FET 135 acts as an open switch and separates the data bus line 220 from the pixel electrode 140. This isolates the potentials on data bus line 220 and the pixel electrode 140 from each other.
When a voltage larger than the threshold voltage is applied to the gate bus line 210, the FET 135 is turned ON (ON state) and has a low impedance between its source 225 and drain 215. The ON FET 135 acts as a closed switch and connects the data bus line 220 to the pixel electrode 140. This transfers the data voltage on the data bus line 220 to the pixel electrode 140.
In the ON state, varying the data voltage on the data bus line 220 varies the voltage applied to the pixel electrode 140. The different voltages applied to the pixel electrode 140 variably turn on the liquid crystal cell 230. Varying the pixel voltage (on the pixel electrode 140) varies the intensity of light as it passes through the liquid crystal cell 155 shown in FIGS. 1, 3, 4, and represented as the LC capacitor 230 in FIG. 2. This results in displaying different scales of gray color on the front polarizer 115 shown in FIG. 1.
FIGS. 3 and 4 show cross sectional and perspective views of a conventional reflective display 300 using an array of reflective liquid crystal spatial light modulators (LC SLMs). The array of FETS 135 are formed on the substrate 125, which is a silicon (Si) wafer, for example. Each FET 135 drives one of the reflective SLMs in the SLM array as described below.
FIG. 3 is a cross sectional view of a single reflective liquid crystal light valve or SLM of the conventional reflection liquid crystal (LC) display 300. The FET 135 is formed between field oxide regions 305 on the semiconductor Si substrate 125. The field oxide regions 305 separate the FET 135 from other FETs or devices formed on the substrate 125. The FET 135 has source and drain regions 225, 215 which are formed in the substrate 125. The source and drain regions 225, 215 are separated by a channel region 310.
Over the channel region 310, a gate insulating film 315 is formed. Illustratively, the gate insulating film 315 is an SiO.sub.2 layer having a thickness which is approximately from 150 to 500 angstroms (.ANG.). A polysilicon gate electrode 205, e.g., having a thickness of approximately 0.44 micron (.mu.m), is formed over the gate insulating film 315.
A layer of dielectric or insulator material, such as an SiO.sub.2 layer 320, is formed over the FET 135 and field oxide regions 310. The storage capacity line 260, also shown in FIG. 2, is formed over a portion of the SiO.sub.2 layer 320 so that the storage capacity line 260 extends over portions of the source 225 and the field oxide regions 305 adjacent thereto. A second SiO.sub.2 layer 325 is formed over the storage capacity line 260 and exposed portions of the first SiO.sub.2 layer 320. The two SiO.sub.2 layers 320, 325 act as inter-layer insulating films.
First and second via holes are formed extending through both SiO.sub.2 layers 320, 325 to expose portions of the source and drain regions 225, 215, respectively. A conductive source line 330 and the conductive data bus line 220, which is also shown in FIG. 2, are formed in the first and second via holes, respectively. The conductive source and data lines 330, 220 extend over portions of the second SiO.sub.2 layer 325 and are electrically connected to the source and drain regions 225, 215, respectively. Illustratively, the source and data lines 330, 220 are aluminum (Al) and have a thickness 335 of approximately 0.7 microns.
A third silicon oxide SiO.sub.2 film 340, acting as an inter-layer insulating film, is formed over the source and data lines 330, 220 and exposed portions of the second silicon oxide SiO.sub.2 layer 325. Over the third oxide SiO.sub.2 layer 340, an optical absorbing layer 345 is formed. The optical absorbing layer 345, which has a thickness of approximately 160 nano-meters (nm), is formed of three layers that are laminated over each other in the following order: A titanium (Ti) layer having a thickness of approximately 100 .ANG.; an aluminum (Al) layer having a thickness of approximately 1000 .ANG.; and a titanium nitride (TiN) layer having a thickness of approximately 500 .ANG..
Laminating these three layers so as to form the optical absorbing layer 345 with a thickness of approximately 160 nm, reduces reflection of light, e.g., having a wavelength from 345 to 700 .ANG., that enters the optical absorbing layer 345 to result in a reflection factor of approximately 25%. The light that enters the optical absorbing layer 345 is shown as arrow 350 in FIG. 3.
In addition, the optical absorbing layer 345 prevents the light 350 from being transmitted to the FET 135 to result in a transmission factor of approximately 0%. The optical absorbing layer 345 improves contrast of images and prevents leakage currents in the FET 135.
A silicon nitride film 355, having a thickness of approximately from 400 to 500 nm, is formed on the optical absorbing layer 345. Next, an Al light reflecting film 140' having a thickness of approximately 150 nm, also shown in FIG. 2 as reference numeral 140 and referred to as the pixel electrode, is formed over the silicon nitride film 355.
A via hole is formed to expose a portion of the source line or electrode 330 of the FET 135. The via hole penetrates through the light reflecting film or pixel electrode 140', the silicon nitride film 355, the optical absorbing layer 345, and the third silicon oxide SiO.sub.2 film 340.
A conducting stud 360, such as a tungsten (W) stud, is formed in the via by a chemical vapor deposition CVD method, for example. The tungsten stud 360 electrically connects the source line or electrode 330 to the light reflecting film or pixel electrode 140'. To prevent electrical connection to the tungsten stud 360, the optical absorbing layer 345 is removed from around the tungsten stud 360.
As more clearly shown in the perspective view of the display 300 in FIG. 4, the light reflecting film or pixel electrode 140' is separated from adjacent pixel electrodes 140'. Illustratively, the reflective pixel electrodes 140' are spaced apart from each other at a specified interval of about 0.5 to 1.7 microns. Each reflective pixel electrode 140', along with its associated FET 135, form a subpixel. For example, three subpixels for red, green and blue components of light form a pixel.
At selected locations of the array of subpixels, pillar-shaped spacers 365 are formed in the space that separates the reflective pixel electrodes 140' from each other. Illustratively, the pillar-shaped spacers 365 are SiO.sub.2 spacers having a width 370 of approximately 1 to 5 microns. The height 375 of each spacer 365 is determined according to the desired cell gap, which is filled with the liquid crystal (LC) 155. The spacers 365 are provided throughout the substrate at specified intervals in order to retain the desired cell gap or thickness d of the LC material 155.
Note, the width 370 of each spacer 365, which is about 1-5.mu., is the same order as the distance of about 0.5-1.7.mu. that separates the reflective pixel electrode 140'. This provides minimum overlap of the spacers 365 with the reflective pixel electrode 140', which in turn minimizes any reduction of the numerical aperture of each subpixel resulting from the pillar-shaped spacer 365.
The counter-electrode 145, which is formed on the glass protect substrate 120, is attached over the spacers 365. The counter-electrode 145 and glass substrate 120 are also shown in FIG. 1. The glass protect substrate 120 is the front portion of the display 300, i.e., the portion facing a viewer. As described in connection with FIG. 1, the counter-electrode 145 is transparent and common to all the pixels. Illustratively, the counter-electrode 145 is an indium titanium oxide (ITO) transparent electrode.
Attaching the counter-electrode 145 over the pillar-shaped spacers 365 forms the cell gap. The liquid crystal (LC) layer 155, in which a liquid crystal material is sealed, is formed in the cell gap between the light reflecting film or pixel electrode 140' and the counter-electrode 145. Orienting films (not shown) are also formed over the pixel electrode 140' and the counter-electrode 145 to orient the liquid crystal molecules.
Illustratively, as shown in FIG. 4, each pixel electrode 140', which defines a subpixel, has a square shape with a side of approximately 17 microns. To form the display 300, the subpixels are arranged in a matrix or array of 1280 rows and 1600 columns, for example.
In the reflective liquid crystal light valve or SLM, comprising the LC material 155 sandwiched between the common transparent ITO counter-electrode 145 and the reflective pixel electrode 140', light 350 entering from the glass protect substrate 120 reflects from the reflective pixel electrode 140'. The pixel electrode 140' also functions as a display electrode for applying a voltage to the liquid crystal layer 155. The FET 135 functions as a switching element for providing a signal voltage from the data line 220 to the pixel electrode 140', when a control voltage on the gate 205 turns on the FET 135, as described in connection with FIG. 2.
An image is projected from the front glass substrate 120, or formed thereon, when the light 350 that enters the front glass protect substrate 120 travels through LC material 155 and reflects back to the front glass substrate 120. This light is reflected from the reflective pixel electrode 140'. Depending on the voltage of the pixel electrode 140', which voltage affects alignment of the LC material 155, the light reflected from the reflective pixel electrode 140', after exiting the front glass protect substrate 120, either passes through an analyzer (not shown) to form an image on a screen, or is blocked by the analyzer from reaching the screen.
The light polarization-rotating properties of the LC material 155 results from varying the direction of the liquid crystal molecules (not shown) in accordance to a voltage applied between the reflective pixel electrode 140' and the transparent ITO common counter-electrode 145. As described in connection with FIG. 2, this voltage is supplied from the data bus line 220 to the pixel electrode 140' when the FET 135 is turned on in response to a control signal on the gate bus line 210 (FIG. 2), which is connected to the gate 205 of the FET 135.
Depending on the voltage applied to the pixel electrode 140', the directors' orientation of the LC material 155 changes. This varies the state of the polarization of light that is incident on the pixel electrode 140', reflects therefrom, and exits the front glass protect substrate 120. Based on this variation of light polarization exiting the front glass protect substrate 120, an image is formed on a screen (similar to a screen 780 of FIG. 7) located between the front glass protect substrate 120 and a viewer.
Various types of polarization dependent conventional LC devices that are fabricated into reflective SLMs. According to the prior art, nematic liquid crystal (NLC) devices are used in reflective light valves for projection displays. These NLC devices require either a linearly-polarized or a randomly-polarized incident light beam.
The present invention concerns primarily with the case where the incident light on an LC device, having a reflective SLM, is linearly polarized. There are various commonly used NLC modes in reflective SLMs that require the incident light to be linearly polarized. The commonly used NLC modes requiring linearly polarized incident light include:
1. electro-control birefringence (ECB) mode with tilted homogeneous alignment; PA1 2. deformation of aligned phase (DAP) mode; PA1 3. hybrid-field-effect (HFE) mode; PA1 4. 63.6.degree. twist mode; PA1 5. hybrid-aligned nematic mode (HAN); and PA1 6. mixed TN (MTN) mode. PA1 M. F. Shiekel and K. Fahrenschon, "Deformation of Nematic Liquid Crystals with Vertical Orientation in Electric Fields", Appl. Phys. Lett., Vol. 19, No. 10, pg. 391 (1971); PA1 F. J. Kahn, "Electric-Field-Induced Orientation Deformation of Nematic Liquid Crystals: Tunable Birefringence", Appl. Phys. Lett., Vol. 20, pg. 199 (1972); and PA1 R. A. Soref and M. J. Rafuse, "Electrically Controlled Birefringence of Thin Nematic Films", J. Appl. Phys., Vol. 43, No. 5, pg. 2029 (1972). PA1 1. the twist angle .phi.; PA1 2. the initial angle .beta., i.e., the angle between the direction 525 of the front LC direction 520 and the incident light polarization direction 545; PA1 3. the pretilt angle .alpha.; and PA1 4. a ratio involving the birefringence, the LC material thickness d, and the incident light wavelength .lambda. as follows: d.DELTA.n/.lambda.. PA1 1. whether the TNLC cell 500 is used in reflective or transmissive displays; PA1 2. whether the polarizer that provides incident light to the TNLC cell and the analyzer that received light from the TNLC cell 500 is crossed or parallel; and PA1 3. whether the cell is operated in the normally-white (NW) or the normally-black (NB) modes.
The first and second modes, i.e., the ECB and DAP modes, are described in the following references:
The third or HFE mode is described in J. Grinberg, A. Jacobson, W. Bleha, L. Miller, L. Fraas, D. Boswell, and G. Myer. "A New Real-Time Non-Coherent to Coherent Light Image Converter--The Hybrid Field Effect Liquid Crystal Light Valve", Optical Engineering, Vol. 14, No. 3, pg. 217 (1975). The fourth or 63.6.degree. twist mode is described in T. Sonehara and O. Okumura, "A New Twisted Nematic ECB (TN-ECB) Mode for a Reflective Light Valve", Japan Display 89, pg. 192 (1989). The fifth or HAM mode is described in J. Glueck, E. Lueder, T. Kallfass, and H.-U. Lauer, "Color-TV Projection with Fast-Switching Reflective HAN-Mode Light Valves", SID 92 DIGEST, pg. 277 (1992). The sixth or MTN mode is described in Shin-Tson Wu and Chiung-Sheng Wu, "Mixed-Mode Twisted Nematic Liquid Crystal Cells For Reflective Displays", Appl. Phys. Lett., Vol. 68, No. 11, pg. 1455 (1996).
Among these LC modes, the present invention primarily concerns with twisted nematic LC cells using nematic LC mixtures belonging to the family of HFE modes and having a positive dielectric anisotropy.
FIG. 5 shows a typical example of a conventional twisted nematic liquid crystal (TNLC) cell 500 used in reflective SLMs. The conventional TNLC cell 500 has a front substrate 505 and a rear substrate 510 to house a nematic LC medium 515 therebetween. The front and rear substrates 505, 510 are parallel to each other.
Between the front substrate 505 and the nematic LC medium 515, there are at least two layers of thin films (not shown). One of the two thin films is a transparent conducting electrode made of indium-tin-oxide (ITO), for example, and the other thin film is a front alignment layer made of organic material, such as polyimide. The polyimide alignment film is located between the ITO electrode and the LC medium 515 and aligns a boundary nematic LC director 520 along a specific direction 525. The boundary nematic LC director 520 is the LC director nearest the front alignment layer.
Similarly, between the rear substrate 510 and the nematic LC medium 515, there are at least two layers of thin films (not shown). The thin film adjacent the rear substrate 510 is a reflective conducting pixel electrode made of aluminum (Al) or its alloys, for example. The other thin film, located between the pixel electrode and the LC medium 515, is a rear alignment layer made of organic material, such as polyimide. This rear polyimide alignment film aligns a boundary nematic LC director 530 along a specific direction 535. The boundary nematic LC director 530 is the LC director nearest the rear alignment layer.
The nematic LC medium 515 has a total thickness, d, and the nematic LC directors twist along an axis 540 which is perpendicular to the planes of the front and rear substrates 505, 510. That is, moving along the axis 540 from the front substrate 505 toward the rear substrate 510, the TNLC directors twist by an acute twist angle .phi. from the direction 525 of the front boundary LC director 520 to the direction 535 of the rear boundary LC director 530. The twist angle .phi. is shown in FIG. 5, on the rear substrate 510, as the angle between the direction 535 of the rear boundary LC director 530 and a projection 525p, on the rear substrate 510, of the front boundary LC director's direction 525.
Upon application of a voltage across the front and rear electrodes, which creates an electric field across the LC material 515, the TNLC directors tilt. For example, instead of being in a plane parallel to the plane of the front substrate 505, the front boundary director 520 tilts by a tilt angle to be in a different plane than the front substrate plane.
Certain TNLC cells have a small pretilt angle .alpha., e.g., below 15.degree.. Note, the boundary LC directors 520 and 530, shown in FIG. 5, are tilted at a fixed pretilt angle .alpha. in the absence or presence of an electric field. This is because the boundary LC directors 520, 530 are strongly anchored on the substrate surfaces 505, 510.
FIG 5 also shows an incident light beam which is linearly polarized along a polarization direction 545 and has a central wavelength .lambda.. The angle between the polarization direction 545 and the front boundary LC director 525 is referred to as the initial angle and is designated as .beta.. The initial angle .beta. is shown on the rear substrate 510 as the angle between the rear substrate projections 525p, 545p of the front boundary LC director 525 and the incident light polarization direction 545, respectively.
The sign of the initial angle .beta. is positive along a clockwise direction from the rear substrate projection 525p of the front boundary LC director 525 to the direction 535 that defines the acute twist angle .phi. therebetween, as viewed from the front of the conventional TNLC cell 500. In the counter-clockwise direction from the front boundary LC director 525, as viewed from the front of the conventional TNLC cell 500, the initial angle .beta. is negative. For example, the initial angle .beta. is positive if it is an acute angle and lies within the twist angle .phi., and negative if it lies at the opposite direction from the front boundary LC director 525 and outside of the twist angle .phi..
In addition to the wavelength .lambda. of the incident light, twist angle .phi., and the thickness d of the TNLC cell 500, another important parameter is the birefringence .DELTA.n of the LC medium 515. The birefringence .DELTA.n is the difference between the refractive index of an extra-ordinary wave and the refractive index of an ordinary wave of the LC medium.
The electro-optical properties of the reflective TNLC cell 500 depend on the following parameters:
To optimize the TNLC cell 500 for the best electro-optical performance for a specific application, the following should be known:
In the NW mode, the display is bright at field-off or low-field regions. By contrast, in the NB mode, the display is dark at field-on or high-field regions.
In summary, there are a total of seven parameters that define a specific embodiment associated with a TNLC cell 500, as shown in FIG. 5, belonging to the family of hybrid-field-effect mode TNLC cells. Table I has columns that list these seven parameters. The rows of Table I compare conventional displays having conventional TNLC cells with a display and TNLC cell of the present invention.
TABLE I __________________________________________________________________________ polarizer/ reflective or .phi. .beta. .alpha. d.DELTA.n analyzer transmissive NW or NB __________________________________________________________________________ Boswell 45.degree. 0.degree. .about.0.degree. crossed reflective NB Bleha 45.degree. 22.5.degree. .about.0.degree. d .about. 2 .mu.m crossed reflective NB or NW Leenhouts 10 to 80.degree. 10 to -60.degree. .about.0.degree. 0.2 to 0.7 .mu.m crossed transmissive NW Velde 0.degree. &lt;15.degree. 0.5.lambda. to 0.6.lambda. crossed or reflective NW parallel Sonehara 63.6.degree. 0.degree. .about.0.degree. 0.33.lambda. to 0.4.lambda. crossed or reflective NB or NW parallel Okumura 0.70.degree. 0.degree. .about.0.degree. 0.2 to 0.7 .mu.m parallel reflective NW present 46 to 62.degree. -6 to 6.degree. &lt;15.degree. 0.7.lambda. to 1.25.lambda. crossed reflective NB invention __________________________________________________________________________
The first row of Table I, referred to as Boswell, shows the seven parameters as described in U.S. Pat. No. 4,019,807, issued to D. D. Boswell et al., on Apr. 26, 1977, entitled "Reflective Liquid Crystal Light Valve with Hybrid Field effect Mode", hereinafter referred to as Boswell. Boswell did not specify the value for d.DELTA.n. However, a value for the thickness d is given as 2 .mu.m, and less then 0.2 .mu.m for .DELTA.d, the variation of thickness d. Boswell has a disadvantage of low brightness, only about 86% as stated on column 10, line 11.
The second row of Table I, referred to as Bleha, is described in U.S. Pat. No. 4,378,955, issued to W. P. Bleha et al., on Apr. 5, 1983, entitled "Method of and Apparatus for Multi-Mode Image Display with a Liquid Crystal Light Valve", hereinafter refer to as Bleha. Bleha is directed to a multi-mode image display without any disclosure or intention to optimize the brightness and the cell gap margin for easy manufacturability. The cell-gap margin, M, is defined as .vertline..DELTA.d/d.vertline., where .+-..DELTA.d is the maximum cell gap or thickness variation from its thickness d. Because cell brightness and the cell gap margin are not optimized, Bleha has the disadvantages of low brightness and poor cell-gap margin.
The third row of Table I, referred to as Leenhouts, is described in U.S. Pat. No. 4,896,947, issued to F. Leenhouts, on Jan. 30, 1990, entitled "Liquid Crystal Display Cell", hereinafter refer to as Leenhouts. Leenhout is designed for transmissive displays with NW operation to generate an electro-optical characteristic with a very slow slope for easy implementation of grey levels. Accordingly, Leenhouts is not concerned with reflective displays operating in the NB mode where it is not desirable to have electro-optical characteristics with a very slow slope. Optimizing brightness for the NW Leenhouts display is different from Optimizing brightness for NB displays.
The fourth row of Table I, referred to as Te Velde, is described in U.S. Pat. No. 4,999,619, issued to T. S. Te Velde et al., on Mar. 12, 1991, entitled "Electro-Optic Display Device for Use in the Reflection Mode", hereinafter refer to as Te Velde. The Te Velde display operates in the NW condition which usually requires much higher operating voltages than NB displays. In addition, the values of parameters .phi., .beta., and d.DELTA.n disclosed in Te Velde are completely different from corresponding values of the inventive display shown in the last row of Table I.
The fifth row of Table I, referred to as Sonehara, is described in U.S. Pat. No. 5,105,289, issued to T. Sonehara et al., on Apr. 14, 1992, entitled "Reflective Type Electro-optical Device and a Projection Type Display Apparatus Using the Same", hereinafter refer to as Sonehara. Sonehara has the disadvantage of requiring high operating voltages to achieve a dark state for NW mode and a poor cell-gap margin for the NB mode. In addition, Sonehara has completely different values for .phi. and d.DELTA.n as compared to the inventive display shown in the last row of Table I.
The sixth row of Table I, referred to as Okumura, is described in U.S. Pat. No. 5,139,340, issued to O. Okumura, on Apr. 18, 1992, entitled "Single Polarizer, Reflective Type Liquid Crystal Display Device with High Brightness and Contrast Ratio", hereinafter refer to as Okumura. Okumura has a completely different value for the initial angle .beta. as compared to the inventive display shown in the last row of Table I. In addition, Okumura uses a single polarizer equivalent to having a parallel polarizer and analyzer to achieve a NW display.
The present invention relates to polarization dependent TNLC cells and displays, e.g., projection displays, using reflective SLMs such as the display 500 shown in FIG. 5. Illustratively, parameters values for the inventive display are shown in the last row of Table I.